The present invention relates to a blow box used in the drying section of a papermaking machine in accordance with the enclosed independent claims and to a method for sealing a pocket with a blow box in the drying section of a papermaking machine. The present invention also relates to an arrangement including a frame construction and a nozzle means for improving runnability in a papermaking machine.
In the drying section of a papermaking machine, the web is transported in a known way by using a single-wire or twin-wire draw. A single-wire draw refers to a draw in which the web runs from one drying cylinder to another supported by one single drying wire over the drying cylinders, the web runs between the cylinder and the drying wire.
A twin-wire draw refers to a draw in which separate upper and lower wires are used for supporting the web as it runs alternately over the upper and lower cylinders. Also in this case, the web runs over the drying cylinders between the cylinder and the drying wire. From the upper cylinder to the lower, or vice versa, the web runs partly unsupported. However, the turning rolls of the drying wires may be fitted so that the wire and the web simultaneously depart from the drying cylinder and so that the wire supports the web a short distance while it passes from one row of cylinders to another.
The drawback of previously known multicylinder single-wire dryers has been that the web is inclined to come loose from the drying wire surface as the wire and the web are transferred via a turning cylinder or roll from one drying cylinder to the next. In this connection problems particularly arise at
the point at which the web and the wire depart from the drying cylinder, along which the web has passed between the wire and the cylinder, and at which point the web tends to depart from the wire and follow the cylinder; and PA1 the entry nips of the wire both on the drying cylinder and on the turning cylinders or rolls below, the web being inclined to depart from the wire at the nips, due to the overpressure induced in the nips.
The diverging of the web from the wire easily leads to the breaking of the web or at least to the web becoming baggy or formation of folds in the web. Thus, the diverging of the web from the wire leads to runnability problems which become emphasized as the speeds of the papermaking machines increase.
It has previously been known to use various blow boxes for improving the operation of papermaking machines. One such component improving the operation is disclosed in the American patent publication U.S. Pat. No. 4,905,380 relating to a blow suction box used in a multicylinder dryer of a papermaking machine. The ejection blow generated by the blow box is used for inducing an underpressure zone in a slot between the drying wire and the blow suction box wall, holding the web on the drying wire as the web passes from the drying cylinder to a turning roll underneath. An underpressure zone is generated downstream of the drying cylinder in the slot limited by the blow box wall and the wire leaving the drying cylinder, by ejecting air from the slot by air blows directed in the opposite direction to the running direction of the wire.
However, a problem with the arrangement described above is to safely seal the underpressure zone induced by blowing from the area remaining outside. In connection with malfunction, the papermaking machines create paper waste which often forms paper clods or causes the web to wrinkle, again causing problems in narrow places in the machine, for example, in the narrow spaces between the blow boxes and wires, cylinders or rolls. Due to paper clods travelling with the web, or other similar bulges in the web, it is not possible to install the blow box at a desired enough short distance from the wire run. Very long safety distances are often required between the wires and blow boxes so that the said bulges could run along with the wire past the blow box without touching it and without damaging its structures or the wire. Safety distances typically are 20 to 50 mm in single wire and 50 to 100 mm in two wire drying sections. When the blow nozzles for the blow boxes have to be placed within the said safety distance, for example, from the wire, the effects of the ejection blow often are inadequate. Neither is the underpressure zone sufficiently sealed.
It is known, for example, from the American patent publication U.S. Pat. No. 4,996,782 to use turning flaps to direct air blows to a given point from which air is to flow through the wire for ventilating the pocket formed by the wire loop. The purpose of the flap is not to seal the slot between the blow box and the wire.
The object of the present invention is to provide an improved blow box and an improved method for sealing a pocket provided with a blow box in the drying section of a papermaking machine.
The object of the invention is especially to develop further a blow box arrangement previously known so that the underpressure effect is further intensified during operation thereof.
It also is a further object of the invention to provide a blow box and method with which it is possible to control the air space between the wire and blow box also during tail threading.
An important object of the invention is further to produce a blow box with which underpressure is generated as close as possible to the element of the papermaking machine moving past the blow box, e.g. a wire, and which may be used both during normal operation and during tail threading.
It is further an object of the invention to produce such an arrangement which makes possible sufficient safety distances for a paper clod or another similar obstruction to pass the blow box along with the wire.
It also is an object of the invention to produce a nozzle for the blow box that is safe to take very close to the wire for generating ejection blowing or pocket ventilation. In this case, the object is especially to produce a nozzle the blow from which may be directed accurately and which may generate the necessary pressure differentials maintaining, however, the operational safety of the structures.
For achieving the above objects, the new blow box and method of the invention in the drying section of a papermaking machine are characterized in what is described in the characterizing part of the enclosed independent claims.
A typical blow box of the present invention used in the drying section of a papermaking machine is, in the said drying section, fitted into the pocket space limited by a wire running from the first drying cylinder to the second drying cylinder, and a turning element, such as a turning cylinder, a turning roll, a suction roll, etc. on this wire run, for ejecting air from the pocket space and for generating an underpressure zone in at least part of the pocket space. At the interface between the desired underpressure zone and the area remaining outside, the blow box is provided with one or several sealing means comprising a sealing element projecting towards the wire at a certain distance "d" from the wire, for forming a seal between the underpressure zone and the area remaining outside the underpressure zone. The said sealing element is preferably joined to the blow box so that the element may, either by pressing or by an actuator, be moved away from the wire to a distance "d'", which is larger than the distance "d".
In this specification and the enclosed claims, a blow box typically refers to box-shaped constructions extending across the web, or to other constructions extending across the web, such as beamshaped or tubular constructions, which may be used for leading ejection air to the pocket or a part of it. A blow box of the invention may also be used for sealing the slot between the end areas of the blow box and the edge areas of the wire for maintaining the underpressure zone at a desired value in the pockets also in the edge areas of the web. The arrangement of the invention may thus additionally, or solely, be used for sealing the edge zone of the pocket, and the rest of the pocket, i.e. in the transverse direction to the web, may be sealed by using some other kind of seal, e.g. a mechanical seal. The sealing arrangement of the invention may also naturally be used more generally in a papermaking machine or some other similar device at a supporting fabric or, e.g. a roll for preventing air from entering into the space between the supporting fabric, or the roll and a blow box, by ejecting air from this intermediate space.
In this specification, the wire typically refers to a dryer wire, felt or some other similar fabric by which the web is supported, for example, as it passes over the drying cylinder.
The invention may be applied to either single-wire or twin-wire drying sections of papermaking machines. In single-wire drying sections, the blow box of the invention, which is fitted into the pocket space limited by two drying cylinders, the wire runs in between, and a turning cylinder underneath, may be used for generating and sealing an underpressure zone in the running direction both in the area of the first wire run from the first drying cylinder, i.e. in the area, which in this specification is called the "entry sides", and also in the area of a wire run extending to the next drying cylinder, i.e. in the area, which in this specification is called the "exit side".
In twin-wire drying sections, the blow box of the invention, fitted into the wire pocket space limited by two drying cylinders, the wire runs in between and the turning roll of the wire, may be used for generating and sealing an underpressure zone in the wire run area extending from the first drying cylinder, i.e. on the entry side. In the area of the wire run extending to the next drying cylinder, i.e. on the exit side, blows causing pocket ventilation may be generated by the blow box.
In an advantageous embodiment of the invention, sealing is accomplished by a sealing nozzle so that air is blown from the blow box through a sealing nozzle, i.e. using a nozzle component fitted very close to the wire. On the entry side, air is blown with the sealing nozzle preferably so that air meets the wire before the wire departs from the cylinder preceding the pocket, the cylinder surface thus preventing the web from diverging from the wire due to blowing. on the exit side, air is respectively blown with the sealing blow nozzle so that air meets the wire only after the nip between the wire and the cylinder behind the pocket has closed; in this case, the blow does not diverge the web from the wire.
The blow box of the present invention is typically fitted at least at a safety distance from the wire, which distance is in single wire drying sections &gt;20 mm typically about 50 mm and in two wire drying sections typically &gt;50 mm up to 100 mm. The sealing component of the invention, such as a sealing nozzle, may during normal operation be brought even to a distance of less than 15 mm from the wire, typically to a distance of about 3-15 mm, preferably 5-10 mm. Thus, it is possible to intensify the underpressure effect generated by ejection between the blow box and the wire, and to seal the intermediate space between the underpressure zone thus formed and the area remaining outside the underpressure zone. The said sealing component is flexible, turnable, or it may otherwise be transferred so that, when a paper clod or some other obstruction is pushing the wire towards the blow box, it may be turned or transferred preferably to a distance "d'"&gt;"d", typically &gt;50 mm, from the wire. A distance 50 mm may be considered sufficient, as it generally allows the paper clods in question to pass past the blow box without damaging it.
The sealing element may again be attached to an actuator which in advance transfers the element a short distance away from the wire, for example, for the duration of tail threading, when it may be expected that wrinkled paper and paper clods run along with the wire more than usual. With the actuator, the sealing element may be transferred, for example, to a distance of about 20-30 mm from the wire for the duration of tail threading.
According to an advantageous embodiment, the sealing element of the invention is formed of one or several sealing blow nozzles joined articulatedly and fitted into a stationary blow box by a link mechanism.
A sealing blow nozzle is typically formed of a slotted blow nozzle extending across the web, or of several sealing blow nozzles fitted sequentially across the web. The sealing blow nozzle is typically fitted to the blow box, in a area which is delimited by the blow box and the entry side of the wire loop, preferably to the beginning of this area as seen from the drying cylinder preceding the pocket, so that the sealing nozzle may be used for ejecting air from this zone and for thus sealing the interface extending across the web between the underpressure zone and the area outside this zone.
The blow box of the invention may principally comprise a uniform main air chamber extending across the web, provided with a sealing nozzle for blowing air and sealing the underpressure zone at least on the entry side of the wire run in a case where the entry side refers to the wire run along which the wire runs to the pocket from the preceding drying cylinder. A second sealing nozzle of the invention may, in a single-wire application, preferably be fitted onto the other side of the blow box for blowing air to the exit side of the wire run, i.e. as it runs from the pocket to the next drying cylinder, so that the entire area of the pocket limited by the blow box and cylinders, wire runs and the turning roll may be brought to an underpressurized state. In an application using a twin-wire draw, a conventional blowing nozzle may preferably be placed onto the other side, i.e. the exit side of the blow box, instead of using an ejecting nozzle, for generating a ventilating blow on this side of the blow box.
The blow box of the invention often is a whole-pocket box which, considering the safety distances, substantially fills the entire pocket space between the drying cylinders and the turning cylinder or a similar part interlaced underneath, and limited by the wire.
On the other hand, the blow box of the invention may also consist of two adjacent blow box parts extending across the wire, with a passage in between, the passage being closable with a closing element and joining the pocket space formed by the blow box and the wire loop with the space outside the wire loop. In this case, it is possible to provide the blow box with at least one blow nozzle blowing air to the passage between the blow box parts for ejecting air through the said passage from the pocket formed by the blow box and the wire loop, and for maintaining underpressure in the pocket.
The blow box parts may be formed of two, principally identically shaped air chambers, which extend across the web and which principally are mirror images of each other.
According to a further embodiment of the invention, the blow box is divided into two different parts, a main box and an auxiliary box. The main box typically is a stationary, conventional blow box extending across the web and fitted into the pocket adjacent to the closing nip of the latter drying cylinder. The auxiliary box, which is fitted adjacent to the wire run coming from the first cylinder, is movable or turnable in relation to an axis transverse to the running direction of the web so that it may be moved/turned from the normal operation position to a different position, e.g. for the duration of tail threading. In a certain position, this auxiliary box may close the passage between the blow box parts and/or form a block for the blow nozzle blowing into the passage. With this arrangement, the advantage may be gained that air outlets from different parts of the pocket may be arranged independent from each other, and thus achieve optimum air removal.
Flexible nozzles are preferably used in connection with the blow box of the invention. Upon meeting a paper clod or some other similar obstruction, they retire so that it is not possible for the paper clod, etc. travelling along with the wire to break or otherwise damage the wire, nozzle or blow box.
The sealing blow nozzle of the invention preferably comprises a stationary frame and an actual, turning nozzle element joined with it with a link. The frame part is firmly attached to the blow box extending across the web, preferably to a cavity the size of the nozzle formed in the box. The nozzle may naturally be joined to other kinds of support elements as well. The frame part of the nozzle, preferably the link in it, is provided with an air inlet joined to the air chamber in the blow box or to some other corresponding part. The actual nozzle element of the nozzle is at its first end attached to the said link of the frame part and to the air inlet in it. The other end of the actual nozzle part comprises a nozzle aperture. The nozzle aperture is connected to the air inlet connected with the first end of the nozzle through an air space inside the nozzle. In addition, the nozzle is preferably provided with a spring or some other transfer element with which the end of the actual nozzle part provided with the nozzle may flexibly be kept pushed towards the wire.
The actual nozzle part is fitted into the pocket on the entry side of the wire preferably so that, upon being pushed towards the wire e.g. by force of a spring, it turns in a sector in the running direction of the wire, as seen from the link. On the exit side of the wire, the nozzle part respectively turns in a sector in a reverse direction to the running direction of the wire, as seen from the link. The nozzle is preferably shaped so that the wall nearest the wire is convex so that, when the wire pushes against the nozzle, it easily slides past the nozzle and does not get caught in it, irrespective of the running direction of the wire. The nozzle aperture is fitted to the other, i.e. the turning end of the actual nozzle part preferably so that it directs the air flow flowing out from the aperture at least partly backwards, as seen from the nozzle part, i.e. partly along the convex outer surface of the nozzle part, outwards from the underpressure zone induced in the pocket.
A considerable improvement is achieved in the runnability of the papermaking machine in the drying section with the blow nozzle arrangement of the invention, as the apparatus of the invention is used for intensifying the underpressure effect in the wire pockets during the operation and as, again, during the tail threading, a very efficient air removal from the pockets is achieved. The underpressure effect generated by the blow box is especially well intensified as the underpressure nozzles themselves are brought as close as possible to the wire and the web running with it. It is possible to hold the paper web attached to the wire on the entry side of the pocket better than previously has been possible, as the pressure zones of the pockets may be controlled by bringing the ejection air closer to the wire and by sealing the underpressure zones from the surrounding air spaces in a better way than before. With the arrangement of the present invention, it is still possible to keep the blow box structures at a suitable safety distance from the wire and to safely move the blow nozzles to the safety distance, desired at a given time, either automatically or by using an actuator.
When the arrangement of the invention is used in a normal single-wire draw, also the amount of air of the turning suction rolls below the drying cylinders may be reduced as the blow box of the invention may be used for intensifying the underpressure effect in different parts of the pocket space.